there are two downloads the ADD-ON and the SDK, you want the ADD-ON, the ADD-ON will NOT have SDK in the file name, try installing and activating BLENDER4WEB ADD-ON if you get errors "clean up" previous attempts of installing BLEND4WEB ADD-ON in your blender add-on folder which is located in a hidden directory (Windows 7) AppData (C:\Users\%YOURWINDOWUSERNAME%\AppData\Roaming\Blender Foundation\Blender\2.73\scripts\addons) , I had to uninstall and reinstall BLENDER for the BLENDER4WEB ADD-ON to work, but I am a round peg and square hole guy and had a lot of baggage after 30 attempts of installation, great plug-in by smart people! Thank You

There are really some ambiguities with addon and SDK installations. For example, if you have installed SDK and will try to remove it the same way as the addon - it will result in some necessary files corruption. This means you can't have both: addon and SDK installed at the same time. For now it is better to choose what type of installation you need and try to not mix two options. But we work on stability and we are looking forward to solve this issue in later releases.